Conventional graphic input systems typically develop electrical signals that represent the position of a writing instrument or stylus on a writing surface. The signals are stored so that information written on the writing surface may be reproduced at a later time. Such systems are used in facsimile transmission and computer data input devices.
One type of graphic input system, such as that described in U.S. Pat. No. 3,886,311 of Rodgers, et al. ("Rodgers et al. '311 system"), employs a sensor tablet that includes a grid of built-in parallel conductors extending in the X and Y directions ("X-Y conductor grid system") along the writing surface of the tablet. DC voltages applied to the conductors provide a writing surface having a unique electrostatic field. In the Rodgers, et al. '311 system, a writing pen uses capacitive coupling to detect a specific position of the pen within the electrostatic field and to provide an output signal indicative of such position.
U.S. Pat. No. 3,904,822 of Kamm, et al., describes a similar system in which the pen includes a pickup coil that is inductively responsive to the X-Y conductors for producing an output voltage that indicates the position of the pen. U.S. Pat. No. 4,289,927 of Rodgers describes an X-Y conductor grid system in which the position of a pen is determined by the X-Y conductors sensing a magnetic field that the pen produces in response to an input signal. U.S. Pat. No. 4,492,819 of Rodgers, et al., describes an X-Y conductor grid system in which the position of a pen is determined by ratios of voltage levels of particular conductors that are produced by an electric field that is radiated by the pen. U.S. Pat. No. 4,616,106 of Fowler, et al., describes an X-Y conductor system in which a pen radiates an electrical signal (or alternatively a signal carried by an electromagnetic wave) and in which the position of the pen is indicated by relative voltage levels associated with the various conductors. Each of these systems is undesirable because of the considerable expense in manufacturing tablets with the X-Y conductor grids and the limitation of writing only on the specially manufactured tablets.
Other graphic input systems have employed sound waves to indicate the location of the writing instrument. U.S. Pat. No. 3,838,212 of Whetstone, et al., describes a system of this type in which a shock wave generated at the tip of a stylus propagates toward two sets of microphones that extend along the entire lengths of the X and Y margins of a writing surface. The position of the stylus is determined by the transit time of the shock wave traveling from the stylus to the microphones. This system suffers from the disadvantage of having an electrical connection between the stylus and the main system circuitry, which electrical connection is awkward and cumbersome for the user. U.S. Pat. No. 4,012,588 of Davis, et al., describes a system that generates at the tip of a stylus a shock wave that propagates toward microphones located at two points. The system determines the stylus position by employing triangulation equations that use the transit time of the shock wave. The system can also generate the shock wave from a third point, in which case the stylus acts as a passive reflector of the shock wave toward the two microphones. The systems of Whetstone, et al., and Davis, et al., are undesirable because sound waves are unsuitable for carrying certain information, such as the color of ink dispensed by the writing instrument.